Traditional high frequency (3.5–30 MHz) antennas, for example, of the mobile type, have an elongate whip or stinger carried at the top of a long mast, typically of about five feet in height. The whip itself may add another few feet to the height of the antenna. An antenna of such a height is often unsightly, if not unworkable, in mobile environments, and thus, it is desirable to provide shorter, more manageable antennas.
The problem is that when an antenna is shortened from its resonant length at the frequency of interest, the feedpoint becomes capacitive, and it becomes necessary to provide a loading coil to add the offsetting inductance. While such will restore the resonance of the antenna, the coil also has resistance which adds losses to the antenna, thereby draining power from the antenna. The more that one wishes to shorten the antenna, for mobile applications, the larger the coil that is required, thereby compounding the power loss problems.
One way to attempt to solve the problem and reduce the size of the coil is to install what is known as a capacity hat. Usually such a capacity hat can replace the whip or otherwise allow for the use of a much shorter whip. Thus, the capacity hat adds horizontal elements to the antenna to add effective length. Ideally, in a capacity hat, the currents in the horizontal sections will offset each other and preserve the vertical polarization of the wave radiated by the antenna. However, to date, there is no known capacity hat which is adjustable to accommodate a wide variety of coil sizes and desired frequencies.